This application claims priority under 35 U.S.C. 119 to German Patent Application No. 20101247.2, filed in the German Patent Office on Jan. 24, 2001.
The invention relates to an apparatus for lettering or marking articles by means of a laser beam.
It has been known for a rather long time to mark articles, particularly packaging materials, by using laser beams. Scanning systems or mask-employing systems may be used for this purpose. In the former case, two rotary mirrors are arranged at right angles to each other to enable the deflected laser beam to sweep over each point of a two-dimensional area. The mask-employing system passes a laser beam through a mask in a pulse-like manner to produce the lettering or marking in a single operation.
It is known to focus the light beam onto the surface of the workpiece with a view to achieving higher power densities and better lettering qualities, as a result. So-called F-Theta optical systems are employed for this purpose. These possess a planar focal area so that the variation in spacing is compensated by the focal area, which normally is spherically symmetrical, particularly at the borders of the plane being lettered.
Another possible way to vary the length of focus in a way depending on the solid angle of the deflected laser beam is to mechanically vary the focal length of the lens system. This requires to mechanically move the lenses, which involves a considerable expenditure in equipment.
It is the object of the invention to provide an apparatus for lettering or marking articles that have a cylindrical surface.
According to the invention, the object is attained by a lens system the focusing plane of which constitutes an area which is curved away from the lens system.
Various possible ways are imaginable to conceive such a lens system. According to an aspect of the invention, the lens system has a first, defocusing lens and a second, focussing lens. According to another aspect of the invention, the defocusing lens has a slightly concave/plane lens and the second lens has a plano/convex or slightly concave/convex lens. According to a further aspect of the invention, the second lens is formed as a cylindrical lens. When cylindrical surfaces are lettered the spacing between the surface and the lens system will vary with the curvature of the cylindrical surface. The cylindrical surface forms a straight line perpendicularly thereto in one direction each. It is understood that the lens system will take account of this circumstance.